The present invention relates generally to an electrochemical device, and more particularly to a device which comprises an electrochemical sensing element including laminated electrochemical cells that use planar solid electrolyte bodies.
There have been known various electrochemical devices which use solid electrolyte bodies, for example as oxygen sensors to detect the oxygen concentration of exhaust gas from internal combustion engines of automotive vehicles. The typical examples of such oxygen sensors include an oxygen sensor which comprises a body of an oxygen-ion conductive solid electrolyte such as zirconia ceramics and which operates to determine the oxygen concentration according to the principle of an oxygen concentration cell. Also known in the art are electrochemical devices such as sensing and pumping elements for hydrogen, nitrogen, carbon dioxide, etc. In such electrochemical devices, solid electrolyte materials have been generally used in the form of a tubular body which has an elongate bore closed at its one end. In recent years, however, it has been attempted to replace the tubular solid electrolyte body with a solid electrolyte body of a planar shape, as disclosed in U.S. Pat. No. 4,334,974, in view of relatively low productivity and high cost of manufacture of solid electrolyte bodies of tubular shape, and from the standpoint of easy assembly of the parts in a planar solid electrolyte body. When such planar solid electrolyte bodies are employed, suitable electrodes are disposed in contact with the surfaces of the planar body of solid electrolyte, and the electrolyte bodies and other parts are assembled in a stacked relationship into a laminar structure constituting an electrochemical cell or sensing element.
In the art of electrochemical devices incorporating a cell of such a laminar structure using planar solid electrolyte bodies, there is also known an electrochemical element which includes an electrochemical pumping cell having pumping electrodes on opposite sides or surfaces of a planar solid electrolyte body, and an electrochemical sensing cell having sensing electrodes on another planar solid electrolyte body. Such an electrochemical element is a co-fired laminar assembly of the electrochemical pumping and sensing cells with a third planar solid electrolyte body sandwiched therebetween. The pumping cell performs a well known pumping function with a suitable voltage applied between the two pumping electrodes. In such arrangement wherein the pumping and sensing cells are constructed in a laminar structure, the voltage applied to the pumping cell may leak toward the sensing cell. This leakage affects an electromotive force generated by the sensing cell, which causes a measurement error, i.e., an erroneous output, of the electrochemical sensing element. Thus, the electrochemical element and device known in the art suffer the above indicated drawbacks.
As a modified form of such an electrochemical sensing element having two electrochemical cells, there is shown in SAE Papers 810433 and 820904 an electrochemical sensing element wherein a cavity formed between the two cells is exposed to a measurement gas outside the sensing element through a leak aperture or pin-hole having a predetermined diffusion resistance to molecules of a component of the gas to be sensed. In such a polarographic sensing element having a leak aperture through which the internal cavity communicates with the outside atmosphere, a soot or similar substance tends to be accumulated in the leak aperture, and change the diffusion resistance of the aperture. Further, the internal cavity requires to have a considerable depth. These factors lead to relatively low response characteristics of the sensing element of the polarographic type.
Further, such element is constructed from two cells combined by glass or the like having an electrical insulation layer therebetween. Such element is apt to be destroyed by a thermal stress which is generated between the glass and the solid electrolyte due to a difference in thermal expansion coefficient.